Concentrations of ozone-destroying gases are down,
but the Antarctic ozone hole is bigger than ever. It turns out
there's more to ozone destruction than just CFCs.

October 2, 2000 -- Scientists have
some good news and some bad news for ozone watchers. Concentrations
of ozone-depleting chlorofluorocarbons
(CFCs) have leveled off in the stratosphere and actually
declined in the lower atmosphere, raising hopes for a recovery
of the ozone layer. That's the good news.

Right: Image of the record-size
ozone hole taken by NASA satellites on September 9, 2000. Blue
denotes low ozone concentrations and yellow and red denote higher
levels of ozone. Notice the "croissant" of high
ozone concentrations formed when the Antarctic vortex blocks
the southerly migration of ozone formed in the tropics. [More
images and credits]

Why are we seeing the worst-ever ozone hole when 13
years of regulation are finally bringing CFC levels under
control?

"The first point is that these processes are really slow,"
said Dr. Richard McPeters, principal investigator for NASA's
Total Ozone Mapping Spectrometer
(TOMS) at the NASA Goddard Space Flight Center (GSFC).

"It takes a long time for the CFCs to get up into the
stratosphere in the first place, so it's going to take a long
time for them to come back out," McPeters said.

CFCs released at the ground diffuse upward through the
lowest layer of the atmosphere, called the troposphere. The vertical
air currents of tropospheric weather help push CFCs up to the
next layer, the stratosphere. Once there, CFCs rise more slowly
because stratospheric air has less vertical air movement.

In fact, it can take a CFC molecule about 2 years after being
released at the ground to make it to the stratosphere where the
ozone is. And it can take decades for it to be converted by sunlight
into a form that is harmful to ozone, according to Dr. Charles
Jackman, an atmospheric modeler at GSFC.

Once a CFC molecule is converted to its destructive
form, it can linger in the stratosphere for a few years before
it drifts back down into the troposphere in the form of hydrogen
chloride (HCl) and is washed out of the atmosphere by rain, Jackman
said.

In 1994, NOAA scientists first measured a decrease in the
amount of CFCs in the lowest layer of the atmosphere. Since these
CFCs would eventually work their way up to the stratosphere --
where the ozone is -- this finding gave hope that CFC concentrations
in the stratosphere would also soon begin to decline.

"It'll be a number of years before you start to see real
reductions in the CFCs in the stratosphere," McPeters said.

Model calculations suggest that ozone recovery to pre-1980
levels could take 20 to 40 years, he explained. "So it's
not something where you'd expect to see a big change this year."

Above: A graph showing the
concentrations of one type of CFC over time. Notice the steady
rise until about 1990 -- three years after the Montreal Protocol
established a phase-out program for CFCs. Concentrations of CFCs
have started to decline. In the graph, "ppt" stands
for parts per trillion, not parts per thousand. [more
information]

Although the concentration of CFCs in the stratosphere appears
to have leveled off, the size of the ozone hole won't necessarily
level off with it.

"What's happening right now is you have the CFCs at a
very high level, and this gives you a background of low ozone,"
McPeters explained. "And then from one year to the next,
whether you have a particularly deep hole or not sort of depends
on the stratospheric 'weather' that you have in the Southern
Hemisphere."

"Because of the overwhelming role of weather in the ozone
hole, it means it's really unpredictable," McPeters said.
"That's what makes it fun to measure ozone -- every year
it surprises us."

This year's record ozone hole occurred largely as a result
of the particularly cold winter in Antarctica, McPeters said.

During
the Antarctic winter, the total or partial lack of sunlight causes
air to drop to very low temperatures. Clouds of ice crystals
called "polar stratospheric clouds" form in the upper
atmosphere.

These ice crystals are bad news for ozone. The crystals provide
a surface for a chemical reaction that changes chlorine in molecules
that do not affect ozone (such as hydrogen chloride) into more
active forms that do destroy ozone.

"That's the accelerator," McPeters said. "If
you didn't have the ice crystals, you would not be seeing the
kind of ozone destruction that you see every year."

A colder winter will result in more extensive polar stratospheric
clouds, greater destruction of ozone, and a larger ozone hole.

Left: An illustration showing
the layers of the atmosphere. Most of the protective ozone layer
lies in the stratosphere, while nearly all weather occurs in
the troposphere.

While higher carbon dioxide concentrations are thought to
cause a warming of the atmosphere's lowest layer (the troposphere),
scientists know that this same carbon dioxide actually causes
the stratosphere to cool down. This cooling can exacerbate ozone
destruction just as a particularly cold winter does.

"Although the magnitude and direction of the lower atmosphere's
temperature change has been very hotly debated for several years,
the cooling of the stratosphere is very clear and not a matter
of question," Newchurch said.

"Its effect in the Southern Hemisphere is to deepen the
ozone hole," he continued. "In the Northern Hemisphere,
this temperature decline and the resulting changes in circulation
(winds) is one of the key ingredients for a possible Arctic
ozone hole."

The Antarctic Vortex

Winds also play a key role in ozone destruction.

The cold air over Antarctica in winter creates a huge "whirlpool"
of fast-moving air circling Antarctica called the "Antarctic
vortex."

This vortex effectively insulates Antarctica from the rest
of the atmosphere.

"It forms up almost as a whirlpool that sits there and
is very stable. It locks in that body of air and it keeps the
outside high-ozone air from coming in," McPeters said.

Most stratospheric ozone is created in the tropics, because
the intensity of the solar radiation that causes formation of
ozone is higher nearer the equator. The ozone is then transported
by stratospheric air currents to the Arctic and to Antarctica.

The strong and stable vortex prevents the migration of ozone
into the stratosphere over Antarctica, exacerbating the low levels
caused by the ice crystal-catalyzed destruction of ozone.

By virtually sealing Antarctica off from the warmer air surrounding
it, the vortex causes temperatures in Antarctica to drop even
lower. Lower temperatures cause the formation of more ice-crystal
clouds and the destruction of even more ozone.

"The (Antarctic) vortex sets up in the (Southern Hemisphere's)
fall and runs all winter," Newchurch said. "The ozone
depletion occurs in the springtime when sunlight becomes available.
Then toward the end of the spring the vortex breaks down, and
over the summer there really isn't a vortex. And then it sets
up again the next fall."

A
similar vortex forms around the Arctic, but "atmospheric
waves" caused by landmasses with high mountain ranges in
the Northern Hemisphere frequently push the vortex off the pole,
allowing warmer air into the Arctic.

The relative warmth of the Arctic is the main reason why a
similar ozone hole doesn't form over the North Pole.

"The weather systems (in the Northern Hemisphere) are
a lot less stable than they are in the South," McPeters
said. "You just don't get temperatures as cold; you don't
get a vortex that will last as long."

Right: Images from a NASA satellite
showing ozone levels over the Arctic (top) and the Antarctic
(bottom) at similar points in each hemisphere's seasons. Blue
indicates low ozone and red indicates high ozone. Notice the
pronounced hole over the Antarctic and the lack of a distinct
hole over the Arctic.

If weather in the North Hemisphere does create a long-lasting
vortex, a mini-ozone hole can be created. This happened in 1997,
but it's unusual, McPeters said.

While this dependency on weather makes year-to-year predictions
of the size of the ozone hole nearly impossible, the long-term
trend can be estimated with computer models.

"If you ask, 'How low is (the ozone hole) going to get?'
Well, we don't know either. Every year we just have to watch
it and see what happens," McPeters said.

"But in the long term, we have quite a bit of confidence
in the models at this point," he said. "In the long
term, it has to get better."

The Global Hydrology and Climate Center is a joint venture
between government and academia to study the global water cycle
and its effect on Earth's climate. Jointly funded by NASA and
its academic partners, and jointly operated by NASA's Marshall
Space Flight Center in Huntsville, Ala., and the University of
Alabama in Huntsville, the Center conducts research in a number
of critical areas.